Summary of Work: The goal of this project is to understand the molecular basis for the normal development of an inner ear. Previously, we have shown that BMP4 (bone morphogenetic protein) is a marker for all the presumptive sensory organs in the chick inner ear. Based on the gene expression results, we postulated that BMP4 may be important in the induction and/or differentiation of sensory organs in the chick inner ear. To learn if BMP4 gene expression is associated with sensory organ formation in inner ears of other species as well, a similar gene expression study was conducted in the mouse. Our results showed that not all presumptive sensory organs of the mouse inner ear express BMP4. We are currently using other candidate genes as well as BMP4 to map the position of all the presumptive sensory organs in the mouse otocyst before differentiation. To address the function of BMP4 during sensory organ formation, recombinant avian retroviral vectors were used for testing a gain or loss of function of BMP4 during inner ear development. The phenotypes of such inner ears are currently under investigation. Besides the BMPs, other genes such as Pax and Msx are expressed in restricted domains in the otic cup suggesting that the axes of the inner ear may be fixed by this stage of development. To test this hypothesis, axes of chick otocysts were reversed one at a time. Operated embryos were sacrificed one week later and their gross anatomy analyzed for sensory organ formation and gene expressions. Our results showed that axial specification in the chick inner ear occurs later than expected and patterning of sensory organs in the inner ear is first specified along the anterior/posterior axis, followed by the dorsal/ventral axis. Our results also showed that the specification of sensory organs and the nonsensory structures in the inner ear may be independent of each other.